I hate to give the old "It Depends" answer, but it really does apply here.
At one time, promo cards were quite valuable, as the supply of them was really limited. They were sent to some dealers who ordered a great deal of product directly from the manufacturers and almost always pictured a super star or highly touted rookie from that particular product. Dealers could also get them by signing up for the VIP attendence packages at large national card shows.
I can recall going to the National card collector's convention in Atlanta with my wife, a friend and his girlfriend. We managed to talk them into waiting in the THREE HOUR line to get the promo bags that the event organizers put together while we roamed the dealer tables. Amazingly, once the general public was able to get these promos, the prices dealers were willing to pay dropped like a proverbial rock.
What the value depends on is:
What release is the promo from?
Who does it depict?
How was the card originally distributed?
My guess is that the promo card will be more scarce that the regular release. If it features a prominent player, then it might have some appeal to collectors of that specific player. But those are only guesses....wish I could offer more than that.